1. Field of the Invention
The present invention relates to an electronic device having a fine shape controlled in the nanometer order, a magnetoresistance effect element having a fine shape controlled in the nanometer order; a magnetic head, a recording/reproducing apparatus, a memory element and a memory array using the magnetoresistance effect element; a method for manufacturing the electronic device; and a method for manufacturing the magnetoresistance effect element.
2. Description of Related Art
A nanotechnology for controlling the structure of a material on the order of nanometers has been valued as a basic technology of information and communication, etc., and research and development have been carried out actively. In recent years, magnetic devices and electronic devices having a fine shape controlled on the order of nanometers have been proposed.
In the field of a magnetic device, since the MR ratio (magnetoresistance change) of a conventional magnetoresistance effect element using a GMR (giant magnetoresistance effect) film (M. N. Baibich et al., “Phys. Rev. Lett., Vol. 61” (1988) p.2472) is only about 10%, magnetoresistance effect elements exhibiting a higher MR ratio have been demanded. A magnetoresistance effect element using a TMR (tunnel magnetoresistance) element, which was proposed with respect to the demand mentioned above, exhibits a high MR ratio of about 50% (T. Miyazaki et al., “J. Magn. Mater., Vol. 139” (1995), pL231). The resistance of the TMR element is inversely proportional to the square of the TMR element width, which is proportional to the track width of an information recording medium. Since the width of the TMR element becomes narrower as the recording density is higher and the track width is narrower, the resistance of the TMR element becomes extremely high. Therefore, it is difficult to apply the element to the fine structured magnetic head.
Furthermore, a magnetic resistance memory (MRAM) using a TMR film has been proposed. However, the MR ratio of the magnetic resistance memory using the TMR film is about 50%, which is insufficient as a memory element.
In such circumstances, there is an attempt to manufacture a magnetoresistance effect element in which a connection portion having a fine shape controlled on the order of nanometers is formed between electrodes and a ballistic conduction capable of allowing electrons to pass through without being scattered is carried out at the connecting portion. In a conventional lithography technology, it is extremely difficult to form a fine shape controlled on the order of nanometers. By using a carbon nano-tube hereinafter, also referred to as CNT) a configuration in which a connection portion of a fine shape controlled on the order of nanometers is formed between electrodes has been proposed (“Nature Vol. 401” (1999), p572, hereinafter will be referred to as non-patent document 3). In this document, a magnetoresistance effect element for electrically connecting two electrodes via a carbon nano-tube is disclosed.
In the field of electronic devices, an electronics device in which the variation of conductance is eliminated has been proposed by controlling the cross sectional area of the wiring by the use of a phenomenon called quantum conductance in which the conductance with respect to the cross sectional area of the fine wiring changes stepwise. The variation of conductance is generated when the conductance with respect to the cross sectional area of the wire is changed continuously. An electronic device having such a fine wiring formed of carbon nano-tube was proposed in Adv. Mater. Vol. 12, 2000, p890, which hereinafter will be referred to as non-patent document 4.
Furthermore, in a nano-contact element using Ni whisker, the MR ratio is reported to be 100,000% at room temperature (see Physical Review B67, 2003, p60401, which hereinafter will be referred to as non-patent document 5)
However, the conductivity of the carbon nano-tube differs depending upon the chirality, so that it is difficult to control the formed carbon nano-tube becoming an arm-chair type exhibiting a property of metal property or a zigzag type exhibiting a property of a semiconductor or a chiral type exhibiting a property of an insulator.
Therefore, in a configuration of the magnetoresistance effect element disclosed in the non-patent document 3, since it is not possible to control the conductivity of a carbon nano-tube formed for connecting two electrodes, the electric property is not stable. If a carbon nano-tube can be formed in the shape of an arm chair type exhibiting a metal conductivity, contact resistance between the carbon nano-tube and the two electrodes becomes extremely high such as 15 to 30 MΩ in the case where the electrode is a Ni electrode. Therefore, this configuration has a problem that the MR ratio of the magnetoresistance effect element is just 10% at an extremely low temperature and that high MR ratio cannot be obtained at room temperature.
In order to obtain an electronic device that is free from variation of conductivity by controlling a cross sectional area of a fine wiring controlled in the nanometer order, also in the configuration having a wiring carbon nano-tube of the non-patent document 4, it is not possible to control the conductivity of the carbon nano-tube as mentioned above. Since there is a problem that the electronic stability is low, it is not suitable for practical use. Furthermore, the resistance is as high as 10 kΩ.
In a nano-contact element using Ni whisker disclosed in the document 5, a nano-contact portion formed of plating has a structure that cannot be controlled. Therefore, it is not possible to use it practically.